1. Field of the Disclosure
This invention pertains to a method and apparatus for treating a photosensitive element, and particularly to a method and apparatus for thermally treating a photosensitive element.
2. Description of the Related Art
Flexographic printing plates are well known for use in printing surfaces which range from soft and easy to deform to relatively hard, such as packaging materials, e.g., cardboard, plastic films, aluminum foils, etc. Flexographic printing plates can be prepared from photosensitive elements containing photopolymerizable compositions, such as those described in U.S. Pat. Nos. 4,323,637 and 4,427,759. The photopolymerizable compositions generally comprise an elastomeric binder, at least one monomer and a photoinitiator. Photosensitive elements generally have a photopolymerizable layer interposed between a support and a coversheet or multilayer cover element. Upon imagewise exposure to actinic radiation, photopolymerization of the photopolymerizable layer occurs in the exposed areas, thereby curing and rendering insoluble the exposed areas of the layer. Conventionally, the element is treated with a suitable solution, e.g., solvent or aqueous-based washout, to remove the unexposed areas of the photopolymerizable layer leaving a printing relief which can be used for flexographic printing. However, developing systems that treat the element with a solution are time consuming since drying for an extended period (0.5 to 24 hours) is necessary to remove absorbed developer solution.
As an alternative to solution development, a “dry” thermal development process may be used which removes the unexposed areas without the subsequent time-consuming drying step. In a thermal development process, the photosensitive layer, which has been imagewise exposed to actinic radiation, is contacted with an absorbent material at a temperature sufficient to cause the composition in the unexposed portions of the photosensitive layer to soften or melt and flow into an absorbent material. See U.S. Pat. No. 3,060,023 (Burg et al.); U.S. Pat. No. 3,264,103 (Cohen et al.); U.S. Pat. No. 5,015,556 (Martens); U.S. Pat. No. 5,175,072 (Martens); U.S. Pat. No. 5,215,859 (Martens); and U.S. Pat. No. 5,279,697 (Peterson et al.). The exposed portions of the photosensitive layer remain hard, that is do not soften or melt, at the softening temperature for the unexposed portions. The absorbent material collects the softened un-irradiated material and then is separated/removed from the photosensitive layer. The cycle of heating and contacting the photosensitive layer may need to be repeated several times in order to sufficiently remove the flowable composition from the un-irradiated areas and form a relief structure suitable for printing. After such processing, there remains a raised relief structure of irradiated, hardened composition that represents the irradiated image.
Photosensitive compositions may contain one or more components that can vaporize or volatilize when the element is heated to the temperature or temperatures necessary for thermal development to occur. The components that can vaporize or volatilize are generally low molecular weight organic compounds, such as monomer. The vapor can condense within a thermal development processor and drip uncontrolled onto different areas of the processor creating a mess within the processor. The formation of condensate in the processor typically depends on usage, with high volume and large plate size being contributing factors. Thus, the vapor and/or condensate from the vapor that remain inside the processor make it difficult to maintain the cleanliness and operation of the processor, and ultimately can damage the thermal development apparatus. In addition, the photosensitive elements can be damaged by the condensate. Condensate dripping onto an image area of the element causes disturbances in the surface of the element that can render the surface unsuitable for printing.
U.S. Publication No. 2005/0084791 A1 by Hackler et al. (published Apr. 21, 2005) discloses a method and apparatus for controlling the vapor and/or condensate created during thermal treatment of the photosensitive element. The vapor is collected at a heating station at or adjacent to where the photosensitive element is heated. The vapor and any resulting condensate are confined within a collection system until they can be effectively removed from the exhaust air. The vapor and/or condensate from the air are removed by converting the vapor to condensate, collecting the condensate, and then treating the condensate for disposal. The vapor and condensate are removed by a separation unit that condenses vapor into droplets, coalesces condensed droplets for collection, and then separates, i.e., by filtration, any remaining droplets (or condensate) from the air. A coalescing filter cartridge array is also mentioned as suitable for removing vapor and condensate from the air. Collected condensate is then drained from the processor or treated for easy disposal. The exhaust air may not, but usually does, need to be treated further to remove the remaining vapor from the air. An external filter or absorption unit may be connected to the exhaust to remove remaining vapor prior to discharging the air from the collection-treatment system or to the outside environment.
However, it is difficult to completely remove the organic material from the air. After collection and treatment to separate liquid condensate droplets from the air, some organic compounds can still remain in the vapor phase in the exhaust air. Oftentimes the remaining vapor then condenses further downstream in the process and still creates a mess for the customer and can damage the processor or exhaust ductwork. Additionally, even effective collection of the vapor and condensate and removal from exhaust air still creates a waste stream that requires handling for disposal.
Oxidation processes (catalytic or thermal) are used in large waste treatment operations in many industries including some printing operations to remove volatile organic constituents from the exhaust. In the printing industry, the organic components of the exhaust arise largely from solvents in the inks and other treatment steps used in the printing process, particularly associated with printing presses. The organic components from the printing process have to be removed from the exhaust air for regulatory compliance.
So a problem arises with effective removal of vapor and condensate collected during thermal development of photosensitive elements from air. It is desirable to remove the organic material from the air to such an extent that if any organic material remains after treatment that it does not impact air handling downstream from the processor to the exhaust. It is also desirable to remove volatilized organic components from the air to minimize handling of waste.